Abstract Elucidating the functions of bacterial secretion systems is important for a complete understanding of host pathogen interaction processes. Secretion systems are complex machines that inject effectors proteins into the host, and in some cases allow establishment of a replicative niche by promoting biogenesis of a cellular compartment that supports bacterial replication. In this project we will determine how the Legionella pneumophila Type IVB secretion system called Dot/Icm functions at the molecular level. This will be accomplished by studying the spatiotemporal dynamics of fluorescently tagged secretion system components using time-lapse video microscopy. Strains of Legionella that have been genetically modified to eliminate individual Dot and Icm proteins, and strains engineered to produce minicells, will be used to determine the structure of the Dot/Icm system by cryo-electron tomography. The overall goal of the project is to use imaging and molecular approaches to characterize what are the spatiotemporal dynamics processes that control Dot/Icm secretion system activity, and to visualize the Dot/icm system so that we have an understanding of the different subassemblies that comprise a functional Type IVB apparatus.